The present invention relates to a method for measuring a vertical density profile of a fluid in a fluid storage tank.
Particularly the present invention relates to a method, a system and a fluid level sensor head for measuring vertical density profiles in a Liquified Natural Gas (LNG) tank.
In an LNG tank the phenomenon of "roll-over" is a potential cause of damage to such tanks. The phenomenon of "roll-over" should be understood as an uncontrolled rising of a superheated fluid layer from the bottom of such a tank, accompanied by rapid vaporization which may overwhelm the tank pressure relief systems. Hereinafter this phenomenon will be described in more detail.
When an LNG cargo is loaded into a tank, underneath already stored LNG, stratification can occur if the cargo is more dense than the original contents. Over a period of time, heat leaking into the tank bottom and the boiling-off of the lighter components at the surface, may cause the densities of the two layers to become equal before the temperatures of the layers come into equilibrium. Convection currents, previously restricted to the separate strata because of the density difference, can now occur throughout the whole tank. As stated above the bottom layer can rise to the surface, accompanied by rapid vaporisation which may overwhelm the tank pressure relief systems with hazardous consequences. Knowledge of the vertical temperature and density profiles can provide operators with information on whether the prevailing conditions may lead to roll-over, so that appropriate operational procedures can be applied.
A conventional method applied in this field is the "bubbler" or gas purge method which comprises the step of measuring hydrostatic pressure, commonly used in the determination of liquid level and density in ambient temperature products. Gas is allowed to bubble through an open-ended tube at a known height in the liquid and a pressure transducer mounted outside the tank measures the gas pressure required to produce the bubbles. This conventional method is not very accurate, because the vertical position corresponding to the measured pressure is ill-defined and depends on the variable mechanism of bubble release.
Thus it is an obJect of the invention to provide an improved method, a system and a fluid level sensor head for measuring a vertical density profile wherein the measurements are made with high accuracy.
Another object of this invention is to provide a method, a system and a fluid level sensor head for measuring a vertical density profile which does not disturb the conditions present in the fluid to be investigated, for example LNG.